A typical Integrated Services Digital Network ("ISDN") call connection offers a user simultaneous access to several duplex digital communication channels, including one or more user data channels ("B Channels"), and one control channel ("D Channel"). The B Channels are intended for the transmission of digital information between network users, while the D Channel is primarily intended to carry signaling protocol messages used for initiating and controlling B Channel calls. These D Channel signaling protocol messages conform to a particular data specification designated Q.931. Limited packets of data called user-to-user information ("UUI") may be transmitted along with the control information under this data specification. The UUI need not be related to a currently established B Channel call.
The capability of the D Channel to carry UUI offers some unique data transmission options to ISDN users. For example, an ISDN user may send a limited amount of UUI to an ISDN recipient via the D Channel by establishing a temporary signaling connection ("TSC"). Naturally, a user is charged for establishing a TSC. Another, more cost effective method would be to transmit the UUI as part of a signaling protocol message for initiating a specific B Channel call. This particular type of signaling protocol message is known as a SETUP message, and the UUI sent along with it is designated message associated UUI ("MA-UUI"). The intended recipient's equipment (referred to as Customer Premises Equipment or "CPE") would receive the SETUP message and the MA-UUI, confirm that a B Channel was associated with the SETUP message, and accept the B Channel call. Normally, the sender would be charged for the B Channel connection, and the MA-UUI would be passed to the recipient at no extra cost. In the alternative, if a user were to send a SETUP message, including MA-UUI, to a CPE which refused to accept the associated B Channel call, the recipient CPE would still receive the SETUP message and MA-UUI. A B Channel call attempt may be refused by a CPE if all B Channels connecting to that particular CPE are in use, or if the CPE is configured to accept only D Channel communications. The practice of sending MA-UUI in D Channel SETUP messages, without establishing a B Channel connection, can provide an economical method of communication for ISDN subscribers.
Unfortunately, in order to perform any type of D Channel data transfer, both the sender and the recipient must be able to access an ISDN and have ISDN compatible equipment at their respective locations--so called end-to-end compatibility. While digital transmission networks are expanding, and the number of ISDN subscribers is growing, the overwhelming majority of existing telephone customers do not have ISDN compatible equipment, and cannot access an ISDN. This effectively limits access to the above described data transmission methods to a very small group of individuals.